Bay scallops: a boom and bust lifestyle

Many species of animals go through dramatic swings in population numbers over time. For some, these fluctuations are related to the dynamics of a natural symbiotic connection such as a predator-prey relationship. A classic example of this is the famous snowshoe hare/lynx model taught to all wildlife ecology students. The lynx numbers follow the hare numbers with a lag in the population upswings and downswings. For other species, it may simply be related to changing environmental conditions that they either do not tolerate well or that they thrive in. This is primarily the case with our panhandle bay scallop populations from year to year. During the time I’ve lived in North Florida I’ve experienced both ends of the spectrum during local scallop seasons. Some years, you can limit-out as fast as you can pluck them from the sea grass bed. Other times, the old adage of “finding a needle in a haystack” comes to mind. Over the past few years we have experienced some of these dramatic swings for various reasons.

Bay Scallop Argopecten iradians

Bay scallops are mostly an annual species, with spawning taking place as water temperatures drop quickly during fall cold fronts. Harvest numbers the following summer are a result of larvae that matured in a single season. Occasionally, you will find an old “mossy-back” that is significantly larger and likely a holdover from the previous season. During spawning, a single scallop can release millions of eggs but very few survive to adulthood and throughout their brief lifespan they are susceptible to many mortality factors.
Predation by crabs, sea stars and several species of marine snails takes a toll but is generally not the driving force in significant declines.

One factor that does have population-level impacts is the amount of rainfall locally. Too much freshwater will create physiological stress and kill scallops over large areas. They can also be hammered by extreme heat or cold events due to their nature of inhabiting relatively shallow coastal waters. Other population pressures may not be so obvious because they sneak up on scallops gradually rather than happening all of a sudden. Factors such as propeller scarring in seagrass beds and siltation from terrestrial runoff or human activities, can have a cumulative effect that gradually degrades the seagrass habitat where scallops live. Another factor that can cause near-extinction of local populations is the occurrence of harmful algal blooms such as red tide. The toxins produced by these marine dinoflagellates will kill fish, marine mammals and shellfish alike. This is what happened to the scallops in St. Joseph Bay during the fall of 2015 when a red tide bloom killed most of the spawning population.
A more recent event in St. Joseph Bay, that put a damper on the 2017 season, was a bloom of a different marine dinoflagellate species known as Pseudo-nitzschia. This organism can produce a toxin known as domoic acid which can cause amnesic shellfish poisoning in humans. Thankfully, it is not expected to harm the shellfish themselves and next season may be a real bumper year. That is, if everything else that can go wrong for a scallop decides to give them a bit of a break. When environmental conditions are good, it is astounding what Mother Nature will provide. Put on your snorkel gear and check it out! For information on seasons and more detailed biology visit the Fish and Wildlife Research Institute’s webpage here. For some tasty recipes check out the Fresh From Florida page here.